The microstructure and tensile properties of RPV cladding material

The microstructural characteristics and transformation behavior of RPV clad material (type 309L) was characterized by optical and transmission electron microscopy. Small tensile test was also performed. As a result of postweld heat treatment Cr-rich carbides (M23C6 type) were uniformly distributed along the initial δ ferrite/austenite grain boundaries and that resulted in the grain boundary migration into the δ ferrite phase. The brittle sigma phase was also introduced. Fine chromium rich carbide particles along the grain boundary provided microvoid nucleation site and that led to final fracture in the tensile specimens at room temperature. At the weld fusion line there existed austenite plus martensite phase that were resulted from excessive dilution effects between RPV steel and clad materials